text:
Was a six-engine 747 ever seriously considered by Boeing?
The Boeing 747 can carry a fifth engine on the side. As the airframe looks quite symmetric, I think that it would not be big work to hang a sixth engine on the side as well.
From here, we seem to be quite near to the six-engine aircraft - a few extra pipes and wires are probably all we need to get these engines turning.
Was a six-engine 747 present at some time of its development history? I think this could be, assuming:

less powerful engines than eventually were available
maybe it could take off with less runway
some special uses with very heavy payload.

There are no six-engine variants mentioned, built or proposed, on Wikipedia.
The expected answer would mention some sources relevant to the design decisions through the history of this aircraft.

For what it's worth, Boeing did make at least one 6-engine aircraft model, though not a 747.  Incidentally, looking around in the categories, there are some truly bizarre looking planes, including some with up to 14 engines...

As a systems guy, "a few extra pipes and wires are probably all we need to get these engines turning" makes me want to sit you down and figure it out. Not saying you're necessarily wrong, but most likely you are.

why stop at six....

The real question to ask it why don't we have a 747 with 4 ge90s yet..

This would be like asking "My car has a cup holder, surely it would be easy to turn the car into a bulldozer."  Heh !

More like a motorboat with one engine working and another hanging in the air over board.

@DatsunZ1 Or a B-52 with 8 ge90s

@Tristan haha the wild thing is only 2 ge90s would be enough thrust for a b52(160000lb vs 1360000 stock with 8 engines:D)Haha if 8 didnt rip the wings off that would be one hell of a takeoff.

The "fifth engine mount" option on the 747 is not designed to handle a running engine. It was an option used only by Qantas as a means of ferrying spare engines to remote locations, where flying a plane for a long distance to a maintenance facility on three engines was not possible. Only four of the Qantas fleet of 747s (totalling more than 60 aircraft) had this fifth engine option.
The mount for the fifth engine was not designed to transmit any thrust the engine would have delivered if it was running, and it fact the engine is partially disassembled by removing the fan to reduce drag (which reduced the loads on the 5th engine mount, as well as drag on the plane as a whole).
Some pictures and videos here: https://www.flightradar24.com/blog/how-qantas-ferried-an-engine-on-the-wing-of-a-747/
To convert this into "a 5 or 6 engined plane" would require a lot more than just "a few pipes and wires". The wing structure would have to be redesigned to handle an extra 50,000 pounds (or more) thrust from each extra engine, plus the extra weight of a proper pylon and nacelle structure. Considering the aerodynamic wing flutter problems with the initial 747 design, sticking another two engines on the wing would most likely have required a complete redesign of the wing.

Pratt & Whitney Canada (PWC) use 2 747SP for engine testing that have an engine mount on the top fuselage and another cradle under wing for different configuration, here's an old article: https://www.747sp.com/pratt-whitney-pw1200g-first-flight/
. Unlike Quantas, these are mount are custom fabricated and fully functionnal for testing purposes. (see https://www.l3t.com/mas/latest-news/2019/06/20/l3-mas-selected-to-modify-a-test-pylon-for-pratt-whitneys-boeing-747sp-flight-test-aircraft/)

In the early 80s, I flew on a British Airways 747 that was taking an engine this way from London to New Delhi. I was wondering if it was normal for the inboard flap to jiggle, even though retracted, when the captain explained what was going on. I suppose BA could have bought or leased this aircraft from Qantas.

"On the image above you can see the specially designed stub wing, which is placed just aft of the co-pilot's seat." – "just aft"? it’s at the other end of the upper deck!

[This answer refers to the original version of the question before it was edited.]

Literally speaking, yes. At least you have thought about it.
Seriously, the target is not to fit as many engines as possible, but how to fulfill the performance and safety requirements with as few engines as possible.
More engines means more power and redundancy, but fewer engines means less cost, complexity, weight, higher fuel efficiency, and a  lower probability of a single engine failure.
The big four engine aircraft are currently losing market share, and  we can observe a transition to two engine aircraft. There are many studies and articles on this topic, for instance "Size matters for aircraft fuel efficiency. Just not in the way that you think" by Dan Rutherford on the ICCT blog.

Sound funny but logically correct. This was not the idea, however. The question has been edited now.

@h22, forgive me, but it is not possible to substantiate with sources that Boeing did not think or consider putting 6 engines on the 747. I'm pretty sure the architects have explored this option at the time. Which kind of source could possibly tell the opposite?

There are books that explore the design history of various aircraft. The soon-to-be-published 'American Secret Projects 3 - US airlifters since 1962 likely contains a section on 747 concepts. At least one of its competitors (from Douglas) had 6 engines.

Why does reducing the number of engines reduce the probability of a single engine failure?

It might be interesting to add an edit note to this answer as it doesn't reflect the question any more and feels slightly awkward

@bogl The probability of a specific, single engine failing remains the same. The probability of any of your engines failing does go up, but then you also care less about that the more engines you have... Perhaps could use some rewording.

I believe I've already explained - I did read carefully, and I find the "short concise wording", as currently written, at best misleading and at worst mathematically unsound. Turning it back on me like I'm less intelligent than you at reading won't help. Thanks.

The probability of a specific, single engine failing remains the same. So, it's not unclear: it's wrong.

@LightnessRacesinOrbit, the word 'specific' is not in my text.

I give up. Have a good day.

The answer is actually Yes, Boeing explored both 5- and 6-engine variants during the design phase of the 747-100 series. So, your first rationale was the correct one.
I read about it in an older book on the 747, written when the plane was only a few years in service. I can't remember the author or title at the moment, I'm afraid, but it had extensive quotes from Joe Sutter and other engineers, and discussed the early teething troubles in detail.
In the early 747 development, one of the biggest problems was the JT9D engine, one of the most powerful engines conceived at the time. The initial prototype version, the JT9D-1, had insufficient thrust even when it was working, which wasn't often. Pratt & Whitney weren't sure how much more power they could eke out of it in time for EIS, and the wear rates were abysmal.
So, uncertain that the plane would have sufficient thrust at the required MTOW for the intended passenger, baggage, and fuel load, Boeing and P&W considered two potential alternatives.

One option was a 5th engine (another JT9D-1) buried in the rear fuselage and fed through an S-duct intake. This was a desperate crutch, since it would increase fuel burn, increase maintenance, and decrease dispatch reliability, but it would guarantee enough power. See note below.
The second option was stranger. They considered installing two JT3D engines alongside the rear fuselage, with a T-tail design. The JT3D was the first-generation turbofan used on the 707, DC-8, and C-141, so just imagine two 707 turbofans mounted like the engines of a DC-9, and you've got the idea. I'm not sure they would have been used in cruise flight.

In the end, the "improved" (but still hideously unreliable and finicky) JT9D-3 engine provided enough power to (just) meet the requirements, and the 5- and 6-engine options were shelved. From there, the -3/-3A variants were replaced by the more powerful and actually reliable JT9D-7 series, and the rest was history. For growth projects after that point, they focused on increasing power -- first with the JT9D-23 planned for the Phase B program, then on with PW4000, etc.
There were several other non-standard engine configurations pondered, too.
Trijets
Note: The center-engine placement for the 5-engine concept came back twice in the 747's history, both in the context of 3-engine 747s. It came up first early in the 747's history, even before EIS. Boeing proposed three-engined variants to compete with the DC-10 and L-1011, as mid-range alternatives, which they dubbed the 747-300 (sometimes, 747-300B) the so-called 747-300B. Different inlet and outlet configurations were considered (the 747-305B and -306B were two of them, plus another with a boundary-layer annular inlet). This was abandoned due to lack of interest. A few years later, with the 747 actually in service, Boeing tried again with the 747-3, the idea being that airlines might now like the fleet commonality it allowed. The 747-3 centered around a proposal for for United Airlines. It would have used an almost straight-through center engine built atop a redesigned aft fuselage. There's no trijet that really resembles it. Most of the trijet proposals were for shorter-body 747s, but unlike the SP, these were designed for shorter-range, domestic and transcontinental routes. The 747-3, for example, would have been less than 200 feet long (and lost 20 feet of wingspan), carried 301 passengers, and only flown 2,600 nautical miles.
In appearance, the trijets varied. Some proposals had T-tails, others had cruciform-style tails like the Sud Caravelle, and some had fairly conventional tails (like the 747-3). One "747-300" proposal even had a stretched upper deck for 44 people, even though it had a shortened body just 174 ft. long and spanning 160 ft. (about 57 ft. shorter and 25 ft. narrower than the 747-100) [FI, 1968-03-28].
I've even seen claims some 747SP development ideas included a trijet with an asymmetrical configuration: twinned engines on one pylon on one wing, a single nacelle on the other. I have no idea how that would have worked safely on takeoff, or even if it's true; that's an internet-forum claim.
Twinjets
Early in the 747 development, there were musings from Boeing about a "big twin" version. This was actually called the 747-200 in some initial articles in Flight International and other publications. There's not a ton of public info to be had, but the references talk about 1,200 - 1,800 n. mi. ranges, 200-240 passengers, but still using JT9D engines. This was conceived as a competitor more to the original Airbus A-300 concept, before the actual A300 [FI, 1968-05-02].
Derated quads
Finally, in the unusual engines department, Boeing considered several reduced-power 747s for US domestic operators. One, a 747-100B with a stretched upper deck (SUD), was used in Japan, but Boeing considered a version powered by four JT9D-7R4E engines (the 767 configuration).
Then, still in 1982, they considered a down-rated 747SP for transcontinental routes. This would have had four engines, too, but they would have been PW2037 or RB211-535E4 models, the same engines as the 757.
Everything else
So, that summarizes pretty much all (known to me, anyway) 747 projects that weren't quadjets, plus the weaker quads. For everything else, Boeing felt that four engines would be powerful enough, if the engine manufacturers developed increased-thrust variants.
The heaviest 747 proposals would have had (or do have) far more power.

1970s stretch studies: JT9D-23, 60,000 lb. thrust range
747 Super, 747-500, 747X: GE90 derated variant (GE90-68?), GE uprated CF6-80E1, PW ADP, PW4168, Rolls-Royce Trent 700, 70,000-75,000 lb. thrust
747-500X and 747-600X: Rolls-Royce Trent 900 or Engine Alliance GP7176, 76,000-80,000 lb. thrust
747X and 747X Stretch: Rolls-Royce Trent 600 or Engine Alliance GP7168 / GP7172, 68,000-72,000 lb. thrust
747-8 Intercontinental: GEnx 66,500 lb. thrust

So, when you think about the change from a 43,500 lb. thrust JT9D-3A, the 747-8I is carrying six engines worth of thrust!


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What is the "departure cable" that tower mentions?
Sometimes the tower will advise aircraft that the "departure cable" is up or down. What are they referring to?
KPDX is one airport where this can be found.

The "departure cable" refers to an arresting gear cable on the departure end of the runway, similar to the one used for landing on aircraft carriers. Airports that often have fighter jet traffic may have an arresting system installed near each end of the runway.
Fighter jets tend to take off and land at higher speeds than other aircraft, and don't have the same braking power that other jets do. The speed brakes may not provide substantial drag, and the engines typically do not have reverse thrust capability. Since there is more room on a runway than on an aircraft carrier, land-based systems take more distance to stop aircraft.
The system is not used for every landing, but is useful to have in certain situations. For normal landings or aborted takeoffs, the system on the departure end will be used to prevent a runway overrun. If an aircraft needs to make an emergency landing, it will use the system closest to the approach threshold. Some cable systems are able to retract into a recess in the runway when not in use, which prevents issues when aircraft roll over it.
When the system is deployed in normal use, the tower will advise aircraft that the departure end cable is up. The departing aircraft will generally be in the air by the time it reaches the cable, but in case of a late rotation or aborted takeoff, the pilots can anticipate running over the cable.
Arresting gear is indicated on the FAA airport diagram as follows:

Source
The cable is marked by 10-foot diameter yellow circles on the runway.

Source: Google Maps

Why's it a departure cable, if it's used when you land?

@cpast I tried to make that more clear in the first sentence.

source: Stack Exchange

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sort: votes


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What is the "yaw damper"?
Some airplanes have a button for the "yaw damper."

What is the yaw damper?
When is it used?
Is it ever required for certification or operation?
How does it work?

The yaw damper is, in simplest terms, an "autopilot" for yaw. When engaged, it operates the rudder (or whatever yaw control method is available in exotic aircraft, like the B-2 flying wing).
Its primary goal is to counteract the typical Dutch roll aerodynamic mode of many aircraft. It also helps keep turns coordinated by automatically providing the necessary rudder inputs to avoid slipping/skidding in a  turn.
In normal aircraft the yaw damper is part of the automatic flight control system (AFCS) or "autopilot." It can usually be engaged on its own without the rest of the autopilot. On the other hand, most autopilots require the yaw damper to be engaged when the rest of the autopilot is engaged (you can have yaw damper without autopilot, but not autopilot without yaw damper).
For large / transport aircraft, the yaw damper is usually engaged throughout the entire flight profile except for takeoff and landing.
It is required equipment (link is to FAA certification regs but similar regs apply elsewhere) in many (most?) transport aircraft, which tend to suffer Dutch roll instability, but flights can sometimes still be made if the yaw damper is inoperable per the minimum equipment list (MEL), with restrictions. This depends on the exact aircraft type and any extra operator policies (company rules). Note that usually an inoperable yaw damper also precludes the use of the rest of the autopilot.
Modern digital yaw dampers are part of an integrated digital autopilot system. They use accelerometers and rate sensors to determine the aircraft's motion. It then runs the numbers through special algorithms to determine what rudder inputs need to be made in order to damp any Dutch roll and to coordinate a turn. It then provides those rudder commands to a servo or hydraulic system which operates the rudder.

The yaw control mechanism for the B2 is still called a rudder but doesn't look like traditional rudders on boats and planes. It's called drag rudders (informally duckerons) and look more like garage doors.

What is the yaw damper?

The yaw damper is a Stability Augmentation System

from Aircraft Control and Simulation

How does it work?

It is an automated system that uses the yaw rate of the aircraft (the aircraft thus need to be equipped with a sensor able to measure it) to command the rudder (the aircraft thus need to be equipped with an actuator capable of deflecting the rudder without pilot intervention) in such a way that the yaw oscillations will be damped faster than naturally.
Most aircrafts are naturally stable around the yaw axis (called "Weathercock stability"), but such natural stability (mostly due to the vertical part of the tail) might not be very damped, meaning that if uncontrolled, oscillations around the yaw axis would continue for a long time before dying off.
The yaw damper aims to reduce the duration of these oscillations.

When is it used?

Whenever there is the need of it.
Usually it should not create problems to the pilot or the safety of the aircraft, but there might be situations in which you might want to disable it, such as when you have to quickly perform a de-crab: a traditional (yaw-rate based) yaw damper would slow down the maneuver.

Is it ever required for certification?

N̶o̶t̶ ̶t̶o̶ ̶m̶y̶ ̶k̶n̶o̶w̶l̶e̶d̶g̶e̶.̶
Thanks to dvnrrs for pointing out that yaw dampers are required for certification of transport aicraft whose flight-test-demonstrated Dutch roll instability exceed certain limits. The relevant requirements are in 14 CFR §25.181.

Is it ever required for operation?

As dvnrrs mentions in his answer, higher level autopilots are usually not engaged if the SASs are deactivated.

@JanHudec Typically yaw dampers are designed using yaw rate as the sensed output and rudder as the input from here [where input and output are referred w.r.t. the system, i.e. the airplane]

Ok, you convinced me. The link looks interesting, too.

"I cannot think for a good reason not to use it." Could it impede an intentional side slip, like a cross-wind landing, perhaps?

@falstro depends. Constant sideslip would not be counteracted, as the yaw rate would be 0. But yeah, it could slow down the de-crab. I will edit the answer.

You'd also want to disable it on the ground. I'm guessing it's only enabled in-flight, i.e. enabled after take-off and disabled before landing.

@falstro again, depends (on how it works, how it has been designed, how it has been implemented). I am pretty sure that the pilot handbook will contain suggestions specific for the aircraft/SAS combination, I would not be sure that is a black-and-white thing.

@falstro I have flown airplanes that specify that the yaw damper should be engaged from engine start to engine shut down (including ground ops) as well as airplanes where it is for airborne use only.

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Why are electric shavers specifically permitted under FAR §91.21
This struck me as a bit strange, 91.21 which covers electronic devices that can be used during IFR flight has some exceptions, one through three make logical sense (not really sure why portable voice recorders are in there though) but number 4, Electric Shavers, is specifically permitted by the regulation. Are that many people shaving under IFR conditions that this needed to be permitted? Is there some historical or practical reason why electric shavers are specifically named in this regulation?

§ 91.21 Portable electronic devices.
(a) Except as provided in paragraph (b) of this section, no person may
  operate, nor may any operator or pilot in command of an aircraft allow
  the operation of, any portable electronic device on any of the
  following U.S.-registered civil aircraft:
(1)Aircraft operated by a holder of an air carrier operating
  certificate or an operating certificate; or
(2) Any other aircraft while it is operated under IFR.
(b)Paragraph (a) of this section does not apply to -
(1) Portable voice recorders;
(2) Hearing aids;
(3) Heart pacemakers;
(4) Electric shavers; or
(5) Any other portable electronic device that the operator of the
  aircraft has determined will not cause interference with the
  navigation or communication system of the aircraft on which it is to
  be used.

I'm willing to bet the reason is similar to why matches are allowed in carry-on baggage but you also need to strip and/or be groped at the security checkpoint. Security theatre.

The question, and the current answer, focus on the crew.  Is it possible that this exception is really for the benefit of passengers?  I'm imagining a businessman on a long flight who is meeting clients at the airport, taking a few minutes in the lavatory to freshen up and look his best.  The other exemptions are also likely to be used by passengers.

This is an educated guess, based on history:
Electric razors were once well known as prone to generating elctromagnetic interference -- the ones available in the 1940s could pretty well blanket a nearby AM broadcast radio receiver.
However, in similar time frames, it was very important for oxygen masks to fit closely on the face, and beard stubble, besides being reportedly very uncomfortable, would compromise the seal (either increasing consumption from the limited store of oxygen available for high altitude flight, or risking hypoxia for the wearer).  Therefore, on long flights, it wasn't at all uncommon for aircrew to shave one or more times before getting where they were going.
As noted in comments, shaving with a blade razor (even the safety razors available when this regulation has its roots) was a bit hazardous in an airplane that might, without any warning, encounter turbulence that could literally knock someone out of their seat if not belted in place.  Wind-up rotary razors existed before 1950, but they were rare, relatively delicate, and expensive.  Electric razors that ran on 115 V were available before 1940 and, with care, would do a good enough job to let an oxygen mask seal well (rechargeable types didn't appear until the late 1950s).
The regulation, then, is to recognize that even with the potential radio interference, an electric razor was specifically to be permitted during flight.

The must have expected long haul flights getting really long soon.

The early commercial regs came out of military practice.  B17 flights England-Germany-England ran long enough that a shave in flight wasn't a bad idea.  Pressurization made this less important after the War, but once something is in the regs, it has to be shown to cause trouble to get it out.

@Peter soon into the 1940s, like the Qantas "Double Sunrise" flight from Australia to India that took (as advertised) over 24 hours...

@Peter Some people grow stubble at 3x the rate others do. And with very stiff hairs a well. I shave a 2nd time before I go out for the night and need to look halfway presentable towards the end of the evening. If I only shave in the morning by midnight I look like most men that haven't shaved for 3 days. Funny thing is that it is mainly the stubble that grows fast. As the hairs get longer (more than about 1-2 mm) it slows down to normal hair-growth speed.

In early unpressurized aircraft, how would the crewmember be able to take their mask off long enough to shave? I can't hold my breath for that long! Did they bring the aircraft down to a lower altitude for a shave break?

@CCTO A B-17 could fly as high as 35000+ feet.  Obviously, you wouldn't try to shave at that height (though you could, for instance, hold your breath while the mask is off and put it back when you need to breathe again).  Flying that high also required some special techniques, because climb rate dropped to near zero above (as I recall) 25000 or so.  It was only done when needed to avoid contrails.

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Engine rear nozzle outside vs inside
I'm not sure if it's called nozzle or something else. I'm talking about the cone shaped part of GA engines at the rear.
Older engines, for example this Trent 700 had the cowling covering whole of the turbine -

Ow newer engines, the rear part of turbine is exposed -

What difference does this have on the engine performance? Is this to accommodate the increasing bypass ratio of modern engines?

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Why some planes are scrapped way sooner than others?
I am browsing the planespotters.net database, and I see plenty of planes in their twenties. Delta, for example, has 117 MD-88 with an average age of 25 years.
While other planes are scrapped with no apparent reason when they are not even 10 years old: when Malev went bust, all its B736s were scrapped, 6 planes that were around 8 years old. https://www.planespotters.net/production-list/search?fleet=Malev-Hungarian-Airlines&manufacturer=Boeing&subtype=737-600&fleetStatus=historic
This is not the only example, there are plenty of aircrafts not older than 15 years that are scrapped, while other planes are still flying in the late twenties.
What are the reasons?

Related: Under what conditions do airliners retire aircraft and sell them to other airlines?

In general, the retirement age of aircraft is around 25 years and most of the aircraft fleet in operation is well over 15 years old.

Source: airfinancejournal.com
There are various reasons for scrapping the aircrafts rather than selling them to other airlines (or other buyers, in general).

Availability of Buyers- People should be ready to buy the aircraft on offer. In this case, the aircrafts were probably offered for sale at a time when tn the economy was in a bad shape and the airlines would've found it difficult to sell them.
Condition of the aircraft- Though the aircraft may be new, it has to be seen if it is properly maintained. In case the aircraft are not maintained properly or had the parts been cannibalized for spares, it would be difficult to find buyers as it takes a lot of money to put the aircraft back into operation.
Sum of Parts- In some cases, the sum of parts of the aircraft may be more valuable than the aircraft itself. Cost of operating an aircraft is quite high and the aircraft parts like engines etc are quite valuable. It would've made more economic sense to scrap the aircraft and sell the parts rather than (trying to) sell the aircraft itself.

Makes sense - who wants to invest that much capital to scrap it after 10 years. They aren't camera hobbyists who need to have the latest and greatest, they're business owners who need to maximize ROI.

Wow, textbook normal distribution!

Just a bit of right skew.

What it boils down to is, does the value of all its parts greater than the value of keeping the aircraft whole and trying to sell or lease it. With your mentioned B736s and I'll add Frontier Airlines A318s which also went to be parted out and scrapped, their parts were far more valuable on the parts market than the owners could get in terms of sales prices or leasing prices. Those types in particular are less economical than many other types, and there is a great demand for spare parts, such that when the airlines went belly up, or decided they no longer wanted the type, the lessors couldn't find anyone who was willing to pay enough to make it worth it, and they decided to scrap them and make their money back from the value of the parts.


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How are the TRACON location IDs determined?
Most of the TRACONs in the US are colocated with a control tower. The location IDs of these match their airport IDs, which makes sense.
There are also several TRACONs that are not located at a control tower. Most of these are designated with a letter and two numbers, such as A11 for Anchorage. The first letter usually matches the TRACON name, but not always. Where did these IDs come from? Does the number signify anything?
Also, there are a few of these non-tower TRACONs that use three letters and no number - what makes them special?

An "up/down" facility is where the tower and TRACON are in the same building and most of the traffic is to/from that airport (typically one class C or TRSA), so they share the identifier. They do provide services for satellite (class D or untowered) airports, but their traffic levels are usually negligible.
A "standalone" facility is where the TRACON is separate building, often on the grounds of a large airport but not the same building, and serves multiple airports (typically one or more class Bs plus any underlying class Cs, plus any satellites) so it gets a distinct identifier with numbers, which can't be confused with an up/down. I've never found a source for where the specific numbers come from; they don't seem random, but there isn't any obvious pattern either.
Recently, the FAA has created "consolidated" TRACONs that merge several neighboring standalone facilities. I'm not sure why the FAA has assigned them all-letter identifiers (e.g. PCT for Potomac Consolidated TRACON), but perhaps it's because there are so few of them (so far) and they cover such large areas that they're relatively well known and there isn't much risk of confusion.

But how are the IDs of standalone facilities determined?

This is anecdotal evidence gleaned from various forum and reddit comments, but I think it makes enough sense that I can justify making it an answer.
FAA Air Route Traffic Control Center (ARTCC) sectors are identified by a name and a number, or only a number. Boston Center, for example, publishes a sector map in a Letter to Airmen; a Syracuse Approach controller trying to contact the Boston Center controller working the "HNK23" sector might say either "Hancock, Syracuse, apreq" or "Two-Three, Syracuse, apreq." The TRACON systems and the ARTCC systems both keep track of ARTCC sectors by assigning each sector a one-letter two-number code, like C23 for "Center, sector 23."
Back when standalone TRACONs first came online, the ARTCC automation system (which may have been HOST or may have been an even earlier system) did not have any possible designation for a sector besides XNN. This is important, because the system needs to know who exactly should be on the receiving end of an automated handoff (where the identification of a radar target is transferred from one controller to another via computer connections, instead of the controllers verbally discussing the location of the target). So the new facilities were added to the center systems as if they were just another sector at an external ARTCC, using the same designator format. By the time facilities like PCT, NCT, and SCT came into existence, the systems had been upgraded and no longer required the workaround.
...Of course this doesn't explain how the system would deal with a normal non-separated TRACON like SYR, so perhaps this answer isn't correct after all. But I have seen it a couple of different places.


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How are AOA sensors tested after working on them?
If a maintenance or replacement has been done for an AOA sensor, would the mechanic be able to test if the sensor now works correctly? Can such a sensor be tested on the ground?
If there is no reading from the AOA sensor available while in the flight, and there is no alert that redundant sensors disagree, I cannot imagine how even pilots could help during the next flight.

Take a look at this page  http://www.dynonavionics.com/aoa-pitot-probes.php  It is easy to visualize how a ground test can be done by changing the angle  that wind is blown at the probe.

I though maybe they use some big fan that blows the wind as needed, but this is in my imagination only so I decided to ask. A pretty serious fan would be required to blow the wind at 200 km/h or about.

It wouldn't be hard to nozzle airflow down to make it go faster. Try this - blow out with your mouth wide open - now purse your lips together and do the same. Feel how much faster the airflow is?

At the manufacturing facility, regulatory requirements mean you need a specialized wind tunnel to perform testing. The big part of the testing revolves around aerodynamics and icing. Testing is done at cold temperature and high altitude conditions and involves, among other things, subjecting the device to solid ice particles and supercooled liquid water droplets in high concentrations.
On the flight line and local maintenance facilities, AOA's are usually tested along with the rest of the pitot-static system with RVSM (Reduced Visual Separation Minimums) test sets such as this one.
This all applies to commercial airliners and private jets, as well as military aircraft. Most general aviation propeller-driven aircraft do not have AOA sensors or indicators, but you can buy third party systems and install them if you wish.


text:
Are checklists always written in English?
Are checklists normally written in English? If pilots are native speakers in another language, is it possible/legal for them to use the translated version of the checklist?
This may be different for the small private aircraft and commercial airliner. Please cover multiple cases when possible.
Soviet Union pilots probably used Russian for checklist. However I ask about open, developed countries (Germany, France, etc) and planes that are used all over the world.

I know for a fact that small GA aircraft POHs are generally translated, including the checklists. No idea about airliners.

In French for the Cessna 172. Note also that checklists may be customized by the aircraft operator.

You suggest that non-English checklists are translations of English. That must often be not the case. I imagine, for example, Brazilian aircraft manufacturers and operators might write their checklists in Portuguese first, then translate to other languages.

This might also be determined by the jurisdiction you're asking about.

Checklists can be in any language.
For general aviation I've seen check-lists in a variety of languages. Sometimes in the local language and sometimes in English - that's in Germany, Spain and Sweden. Spain in particular seemed to have everything in Spanish. From colleagues I've heard that France is similar, most GA check-lists are in French.
For commercial aviation it varies by airline. Air France, for instance, publishes its check-lists in French (an example of these can be seen in the Air France 447 final report)

It is definitely common here in Germany to use German checklists (German is not an ICAO language).
see http://www.motorflugschule-egelsbach.de/Downloads/Diverses/C172_R04.pdf for example.
However since many systems and acronyms used have English terms anyways it boils down to a combination with English.


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Why is the 787 tail cone made out of titanium?
In this question there is a drawing of a 787 that shows that the tail cone is made of titanium.
Why is such a heavy-duty material like titanium required for the tail cone?
I do not see how there could be any significant loads or reasons for excess temperature there.

It is probably to increase stiffness of the rear control surfaces- the elevators and rudder can bear enormous forces in their design envelope. Failure at that point would be a guaranteed disaster. Note the statements about use of materials in the 787 are bit misleading it is all high grade aeronautical alloys each with incredibly high strengths.

That is the stinger, not the entire tailcone, which extends up to about the location of the last window shown. And the graphic is not indicating that the stinger is made of titanium. Rather, it is giving the overall percentage of titanium used in the entire aircraft. Each arrow does point to locations where the material is used heavily, but other materials are also used in these regions. While I know about Cessna jets, not the 787, I can say with high confidence that there is likely far more Al in the stinger than Ti. Gypaets is correct about why Ti is used there.

@PaulSinclair -- the color coding in the diagram does suggest that it's indicating the material used in each surface. For example, it indicates in purple 10% steel (mainly for landing gear), but since the landing gear is not visible, there's no purple on the aircraft diagram.

Whenever you see titanium being used on aircraft, look for heat sources. The metallic alternative aluminum is usually lighter and much cheaper.
In most commercial aircraft the APU (Auxiliary Power Unit) is in the tail, behind the rear pressure bulkhead and the empennage.

(modified from aerospaceweb.org)
The APU is a small turbine and its exhaust gases are very hot (I don't have exact data and it depends on the APU model, but the exhaust temperature should be somewhere in the range between 400°C and 800°C). Aluminium has not only a low melting point (660°C), but its mechanical properties degrade fast once it gets hot. On the other hand, titanium has a higher melting point (1668°C) and still performs very well at high temperatures without significantly loosing strength.
While carbon fibers don't lose their mechanical properties at high temperatures, the resin types used as a matrix are very far from being able to withstand these temperatures. Even PEEK, probably the best resin for use at "high" temperatures, has a glass temperature of 143°C (melting point at 343°C). This makes resin-based composites inappropriate. An alternative would be using ceramic materials as a matrix (CMCs). However, the simple geometry of the tail cone probably makes the production with titanium cheaper than with CMCs.
Here's an image of the cone (source: flight.org):

it is not necessarily only melting points - aluminium performs really poorly at the "lower" elevated temperatures (~400ish) , softening, grain growth / segregation of alloying materials, diminished fracture strength, and not to mention increased corrosion, as the oxide layer tends to "peel off". Titanium on the other hand, is a champion of high temperature performance. (material scientist nitpicking aside, +1 for you)

Great answer, thanks.  I wonder how composites would handle these temperatures??

@StianYttervik Of course, you're right. Nitpicking is welcome! I've expanded the answer with it (and the questions of Fattie and BruceWayne).

In addition to heat from the turbine exhaust, I was wondering if fire containment and extinguishing might be an issue. This article is not 787-specific, but it suggests that fire may well be an issue: http://www.boeing.com/commercial/aeromagazine/articles/2010_q4/3/

@NoahKrasser See the article I linked to in my other comment for an example of Titanium used as heat shields and fire seals on engine pylons.

Nice. Some more composite nitpicking: Resins are precursors for polymers,  and mostly thermosets at that. Although thermoplastic stuff (like PEEK) is also often put under the resin nomer, this is not correct. There are thermosets that work up to more than 350°C, which are used for exhaust parts (https://www.compositesworld.com/articles/are-high-temp-thermosets-ready-to-go-commercial), but it seems that Ti is the better option. CMCs are way too expensive for such large parts in a commercial setting, and while tougher than pure ceramics, they are much more brittle and less tough than Ti alloy.

Heat is not the only reason to use Ti. It's used for the leading edge of carbon fibre fan blades to give them impact resistance.

Another plus for titanium is its much better electrochemical compatibility with the graphite-epoxy structure ahead of the tailcone. Being at the extreme end of the fuselage, it must be electrically connected with the fuselage structure for lightning protection.


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Why this formula says jet engine is efficient at any speed?
Wikipedia gives the following equation for the efficiency of the jet engine, here:
$$\eta_p = \frac{2}{1 + \frac{v_e}{v}}$$
Where V is the aircraft speed and Ve is the exhaust speed. It is backed by some reference to the book I have no access to so cannot clarify what do they want to say by that.
I am still deeply puzzled, how it could be the efficiency increases all the time with the increase of the aircraft velocity, even after it exceeds the exhaust speed many times. I think I have read somewhere that the efficiency declines and not keeps growing after the aircraft speed approaches exhaust speed.

If you look at the plot in the Wikipedia article, you will see that the propulsion efficiency for jet engines is only defined up to $ v = v_e $.

This is the result from the simple momentum balance. In order for the propulsor to produce thrust, the exit speed after the propulsing element ($v_e$) must be higher than the incoming speed ($v$):
$$T=\dot{m}(v_e-v)$$
where $\dot{m}$ is the total mass flux through the propulsor.
So the correct way to read the efficiency formula is: the closer the exit speed is to the incoming speed, the more efficient it is! Therefore, the more mass flux you can generate at a smaller speed difference, the more efficiency you can attain without sacrificing the thrust. This is the overarching reason why bypass ratio makes engines more efficient.

And this is why modern jet engines are so huge: The fan is much bigger than the actual engine (the compressor, firebox, and turbine) which allows it to push a large amount of air back "gently".

This is the so called Propulsive efficiency!
It is the main factor for new engines. The smaller the difference between the flight velocity and the jet stream velocity, the higher the Efficiency. Therefore you want a big Fan, which produces a small velocity difference (on big crosssections).
https://en.wikipedia.org/wiki/Propulsive_efficiency


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What is special about the ICAO Caribbean region for VFR climb/descent?
In the FAA JO 7110.65 that provides rules for air traffic control, the following limitations are listed for VFR climb and descent:

8−8−5. VFR CLIMB AND DESCENT
a. In the Houston, Miami, and San Juan
  CTAs, IFR flights may be cleared to climb and descend in VFR
  conditions only:
  1. When requested by the pilot; and
  2. Between sunrise and sunset.

The key part here seems to be the second condition, as section 7-1-2 just lists the first one.

7−1−2. VFR CONDITIONS
a. You may clear aircraft to maintain “VFR
  conditions” if one of the following conditions exists:
  1. The pilot of an aircraft on an IFR flight plan requests a VFR climb/descent.

So what makes the Houston, Miami, and San Juan areas special that VFR climb/descent must be during the day? Section 8 is the Caribbean ICAO region, which seems to only include the oceanic sections of Houston and Miami. But this rule doesn't specify "oceanic" and none of the other oceanic areas seem to have this rule.

Multiple thunderstorms in the area, perhaps?

The Gulf and Caribbean have many light planes and helicopters at low levels who may wish to avoid clouds despite being IFR. Other oceanic regions are almost exclusively jets above the clouds.

It looks like by default, night VFR isn't allowed in oceanic airspace. The 8-8-5 text you quoted appears to be an exception that does allow night VFR in certain specific Control Areas (CTAs) that are in US airspace.
This is from the US AIP 7.1:

1. IFR/VFR Operations
1.1 Flights in oceanic airspace must be conducted under Instrument Flight Rule (IFR) procedures when operating:
1.1.1 Between sunset and sunrise.
1.1.2 At or above Flight Level (FL) 055 when operating within the New York, Oakland, and Anchorage Oceanic Flight Information Regions
  (FIRs).
1.1.3 Above FL180 when operating within the Miami and Houston FIRs and in the San Juan Control Area. Flights between the east coast of
  the U.S., and Bermuda or Caribbean terminals, and traversing the New
  York FIR at or above 5,500 feel MSL should be especially aware of this
  requirement.

1.1.1 essentially says "no night VFR"; 1.1.2 and 1.1.3 say "except in US airspace".
As to why, many countries don't allow night VFR at all and I suspect that it's generally not allowed in oceanic regions to simplify international operations and/or comply with ICAO requirements. Trying to remember which countries allow night VFR as you fly along an international route is probably impractical, especially in the Caribbean. But, since the US does allow night VFR, a VFR climb/descent is possible if the specific airspace in question is US airspace.
(I couldn't find any source on why these rules exist so this is all just my own explanation based on what I could piece together. Someone else may come up with something better.)


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What qualifies as "overwater"?
I noticed that the saftey cards on a recent domestic US flight were the "overwater" version. Two questions about this:

What qualifies as "overwater"? Is there an official (i.e., FAA) definition?
Do airlines really have certain aircraft with booklets that aren't "overwater"? Are they prevented from flying certain routes without the right safety booklets?

Flew Ft Lauderdale to New York last week and wondered the same.. the safety card in a A319 (OW)!

I think that 14 CFR Section 91.205(b)(12) defines "overwater".

(12) If the aircraft is operated for hire over water and beyond
  power-off gliding distance from shore, approved flotation gear readily
  available to each occupant and, unless the aircraft is operating under
  part 121 of this subchapter, at least one pyrotechnic signaling
  device. As used in this section, “shore” means that area of the land
  adjacent to the water which is above the high water mark and excludes
  land areas which are intermittently under water.

Then 14 CFR Section 121.573 outlines the passenger briefing requirements for flights with overwater operations:

§121.573   Briefing passengers: Extended overwater operations.
(a) In
  addition to the oral briefing required by §121.571
(a), each
  certificate holder operating an airplane in extended overwater
  operations shall ensure that all passengers are orally briefed by the
  appropriate crewmember on the location and operation of life
  preservers, liferafts, and other flotation means, including a
  demonstration of the method of donning and inflating a life preserver.
(b) The certificate holder shall describe in its manual the procedure
  to be followed in the briefing required by paragraph (a) of this
  section.
(c) If the airplane proceeds directly over water after takeoff, the
  briefing required by paragraph (a) of this section must be done before
  takeoff.
(d) If the airplane does not proceed directly over water after
  takeoff, no part of the briefing required by paragraph (a) of this
  section has to be given before takeoff, but the entire briefing must
  be given before reaching the overwater part of the flight.

Lastly, the requirement for printed safety booklets is from 14 CFR 121.571(b)  And to answer your last question, it needs to have only information pertinent to the flight, so if the airplane is not going to fly overwater at all, it doesn't need to have it.

(b) Each certificate holder must carry on each passenger-carrying
  airplane, in convenient locations for use of each passenger, printed
  cards supplementing the oral briefing. Each card must contain
  information pertinent only to the type and model of airplane used for
  that flight, including—
(1) Diagrams of, and methods of operating, the
  emergency exits;
(2) Other instructions necessary for use of emergency
  equipment; and
(3) No later than June 12, 2005, for Domestic and Flag
  scheduled passenger-carrying flights, the sentence, “Final assembly of
  this airplane was completed in [INSERT NAME OF COUNTRY].”

Note that part 121 of the FARs only applies to "air carriers and operators for compensation or hire" (and similarly 91.205(b)(12) only to "for hire" operations), though it's a good idea for part 91 operators to carry flotation devices & ensure their passengers know how to use them if the flight will be over any significant amount of water.

:) Capt Sulley's flight wasn't overwater, he just didn't want to attempt to land in downtown Manhattan...

@voretaq7 yeah, there is a lot of wisdom in stuff required for "for hire" operations that not-for-hire operators can do to enhance safety.


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What is the purpose of multiple autopilots?
I have read that some airplanes have multiple autopilots, specifically airliners.
What is the reason for this, and how does that system work?

Among others, one reason is higher cosmic radiation at high altitude, e.g. compared to sea level, neutron flux is ~4X at 1.5km (Denver's height) and ~300X at 12km(where airplanes fly). This causes soft errors in electronic components. See this survey paper for reference (Section 2.3).

Redundancy, particularly during autoland operations.
How it works depends on the specific airplane type. In the most advanced systems with triple (or more) flight guidance computers (FGCs), a "voting" concept is used during critical operations like autoland. All 3 computers calculate their commands independently, and vote on what to do. The middle value, or sometimes the average of the 2 "most similar" values, is then passed to the flight control actuators. In this way, if any one of the computers fails and starts producing garbage commands, those erroneous commands will not be selected (they are automatically rejected by the voting scheme). A monitoring system can also detect a failed computer in this way and take it offline, allowing "fail-active" functionality — an autoland can continue on the 2 remaining functional computers.
Less complex aircraft, and aircraft without autoland, still sometimes have 2 FGCs, but do not support fail-active operation. In these systems only one of the two computers is active or selected at any given time. A transfer switch in the cockpit allows the pilot to transfer control from one computer to the other if one fails.
And it's not just airliners. Business jets commonly also have dual FGCs, and in recent times even small general aviation airplanes like the Diamond DA-42 have dual systems (Garmin G1000 in dual-GFC700 configurations).

This photo (click for high res version) shows a Cessna Citation CJ business jet with dual Garmin autopilots. At the top of the PFD, notice there is a small green arrow pointing left; this indicates the left FGC is currently selected. The photo also shows the XFR (transfer) button on the mode select panel, top center, just underneath the master caution/warning annunciator panel. Pushing this button would switch to the right (#2) FGC and change the arrow on the PFD to point to the right. These indications are fairly representative of other dual-FGC systems.
Here's another photo of a Hawker 800 with Rockwell Collins Pro Line 21. You can see the same arrow on the PFDs. In this aircraft the transfer switch is labeled "AP XFR" and is again on the autopilot panel just underneath the glareshield at the top of the panel toward the left hand side.
And finally a photo of a 767 with 3 FGCs and full autoland capability. It's a bit hard to read in this photo, but just above the autopilot disengage bar (the "spacebar" looking thing on the right side of the mode select panel just under the glareshield) the 3 buttons are labeled "A/P ENGAGE", "L" "C" and "R" for the left, center and right FGCs. During an autoland all 3 would be selected and illuminated.

if any one of the computers fails and starts producing garbage commands
As the FGC is a piece of software, and all software is prone to bugs and errors, it is likely that any given FGC will produce garbage from time.  The voting system prevents the plane from crashing when this happens.

@Jumbogram Actually it's extremely unlikely. FGC and other flight-critical software is painstakingly designed and tested under rigorous software development guidelines. It is then extensively analyzed and tested prior to flight certification. Erroneous commands due to a software error are extremely unlikely (but, yes, technically possible, and designed to be tolerated via redundancy).

@Jumbogram Far more likely is that the garbage data is the result of a communication issue with other avionics (e.g., bad navigation data being received), or electrical/RF interference, or damage from a power surge or something.

@dvnrrs Obligatory CS/Engineering lecture case study -- always expect failure, hence the voting system :-)

@voretaq7 Yep, and note that the incident you refer to led to the creation of the IEC 62304 software standard, which is probably less restrictive than DO-178B used in aviation. I'm a software engineer myself and I assert that although software bugs have occurred in safety-critical areas in the past, they are still extremely unlikely in flight-critical avionics due to the rather extreme levels of caution in use today.

I can't see the "small green arrow" at the top of the PFD even in the enlarged photo. It must be about 1 pixel wide?

@RedGrittyBrick Aww come on, it's at least 3 pixels :-) Just under the purple blur. Sorry, best resolution I could find.

@Jumbogram Most avionics software failures have been found to arise from errors in the design and specification, or a failure to identify possible corner cases.


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How are airspace fixes named?
Aircraft use nav fixes as waypoints or reporting points along their routes. I have noticed some interesting combinations.
Some make sense, like LUCKI to LYNDI on the LYNDI arrival into San Diego International, known as Lindbergh Field, or KSINO LUXOR on the GRNPA arrival into Las Vegas.
Some are more interesting, like HIMOM KALME on the FRNCH arrival into Denver, or ASTAH LVSTA ADYOS on the ADYOS departure from Albuquerque.
Some get downright suspicious, like ITUNE MUSCC DWNLD on the LOWBO arrival into Albuquerque.
So my question is: how do these points get named? These combinations are memorable, but are they also allowed to be sponsored? Or is this a case of "Any resemblance to actual entities is entirely coincidental."

some interns at FAA maybe

lol @ ITUNE MUSCC DWNLD

For an Airplane! sequel.

I find it interesting that some of the fixes on the KMSP ILS 35 approach are JAMEZ, OBERR, and STARR. Jim Oberstar was chairman of the House Transportation and Infrastructure Committee at the time the approach was commissioned.

In the US, they are assigned by a division in the FAA called Aeronautical Information Management (AIM), per FAA Order JO 7400.2J (Procedures for Handling Airspace Matter), section 3-3-2:

a. Service area office are responsible for assigning and changing names of NAVAID and aeronautical facilities, and must follow the instructions contained
  herein and in FAAO JO 7350.8, Location Identifiers, Chapter 1.
b. AIM is responsible for issuing five−letter names for radio fixes, waypoints, marker beacons, and compass locators. Five−letter names must be issued by AIM to the Terminal Procedures and Charting Group, Major Military Commands (MAJCOM) and Air Route Traffic Control Centers (ARTCC) for future assignments.

It's conceivable they accept suggestions from ARTCCs or other entities. I don't know. I doubt paid sponsorship is approved but have no first hand knowledge there. Suggestions are probably casual and intormal.
They are often quite amusing, like ITAWT ITAWA PUDYE TTATT. Very often enroute fixes are named for small towns or other points of interest near the fix.

A particularly interesting collection of waypoints on the FRDMM STAR into Washington: http://flightaware.com/resources/airport/ADW/STAR/FRDMM+TWO+(RNAV)/pdf

Linguist and writer Deb Fallows has an interesting article citing some of the more unusual or noteworthy names: http://www.theatlantic.com/national/archive/2013/10/itawt-itawa-pudye-ttatt-the-secret-language-of-the-skies/280598/ and http://www.theatlantic.com/national/archive/2013/10/eatnn-ttuna-snwch-hold-at-mayyo-more-on-the-secret-language-of-the-skies/280662/.

For all those who didn't the the ITAWT IWATA PUDYE TTATT line... Tweety: https://www.youtube.com/watch?v=jHfpWqiU4ss

You will find some more funny combinations here: Air Navigation Name Nonsense

As well as the FAA's part mentioned in dvnrrs's answer, they are also part of ICAO's 5LNC (Five-Letter Name-Code) system, which records available pronounceable 5 letter codes (they don't have to be words).
While these are meant to be unique, there are several cases where different countries have duplicate names from legacy naming systems. As of February 2014, there are 130 codes that are duplicates of others in other countries.
Usually, there is usually some form of selection to prevent similar-sounding names from being used as a 5LNC in the area. For example, the UK Civil Aviation Authority states that (emphasis mine):

Where a significant point is required at a position not marked by the site of a radio
  navigation aid, the significant point shall be designated by a unique five letter
  pronounceable ‘name-code’ (5LNC). This name-code designator then serves as the
  name as well as the coded designator of the significant point and shall be selected so
  as to avoid any difficulties in pronunciation by pilots or ATS personnel when speaking
  in the language used in ATS communication. The name-code designator shall be
  easily recognisable in voice communications and shall be free of ambiguity with those
  used for other significant points in the same general area. The name-code
  designator assigned to a significant point shall not be assigned to any other
  significant point. (Reference B)

The ICAO 5LNC database link appears to be broken.

@MichaelKjörling it seems public access has been removed unfortunately.

Note, my answer that follows is per the US, and having been involved and discussed various naming of fixes and routes. Paid sponsorship doesn't happen. Usually it's the ATC facility that suggests names, and check the existing and reserved names lists.. For SIDs/STARs, the divide is usually the TRACON names the SIDs and the Center names the STARS. For random fixes that don't have to be as pronounceable(lots of RNAV approach fixes/intermediate fixes on arrivals), they can just be randomly assigned as long as they stand a chance of being able to be pronounced.
Controllers have a sense of humor and will see what they can get away with in terms of naming fixes. They have been named for people, both controllers, or people from the area. They can be a joke like the above mentioned ITAWT ITAWA PUDYE TTATT. Though a lot of the more historical fixes on Victor and Jet Airways are named for towns or features they're near.


text:
How does IDENT work?
In order to help ATC identify an aircraft on their screen, they may ask the aircraft to "ident." The pilot presses a button on their transponder, and the controller can verify that the aircraft they are talking to is the one they are looking at on the screen.

In what situations does ATC ask for ident, and is it ever required by procedure?
What does the transponder actually do in this mode?
What does the controller see on their screen?
Does it depend at all on either the code or the mode the transponder is currently set for?

(Note: This answer is tailored to the U.S. / FAA, because that is what I'm familiar with. All the citations and references are FAA materials. If anyone has any tips on possible international differences, please let me know and I'll edit them in. That said, I believe the descriptions I give are fairly consistent across the world, even if the FAA reg citations don't directly apply.)

In what situations does ATC ask for ident, and is it ever required by procedure?

Generally, "ident" is used to help a controller identify an aircraft's secondary radar (transponder) return. By asking the pilot to "squawk ident," the controller can ensure that the aircraft he/she is talking to matches the radar target he thinks is that aircraft.
It can also be used for communication, to allow a pilot to respond if their radio transmitter has failed.
Specifically:

It is a method to allow a controller to identify an aircraft on radar, per section 5-3-3 of FAA Order 7110.65 (pdf):

When using only Mode 3/A radar beacon to identify a target, use one of the following methods:
a. Request the aircraft to activate the "IDENT" feature of the transponder and then observe the identification display.

For lost communication, per section 10-4-4 of 7110.65:

Take the following actions, as appropriate, if two-way radio communications are lost with an aircraft:
c. Attempt to re-establish communication by having the aircraft use its transponder or make turns to acknowledge clearances and answer questions. Request any of the following in using the transponder:
1. Request the aircraft to reply Mode 3/A “IDENT.”

This last scenario is common when experiencing radio failures. If ATC suspects that you might be able to hear them but not to talk back, they might say something like "Cessna 12345, if you can hear me, squawk ident." If you reply by pressing the IDENT button, they can then issue you further instructions and ask you to acknowledge those instructions by pressing IDENT again.

What does the transponder actually do in this mode?

In normal operation, every time the transponder receives an interrogation from a ground station, it replies with a data block containing certain pieces of information. This data block contains:

The encoded pressure altitude (for Mode C transponders)
A 12-bit identification code (this is the 4-digit code you select in the cockpit)
A single extra bit for the "ident" flag

When you push the IDENT button in the cockpit, the transponder sets that extra "ident" bit in its reply data. Anytime the transponder is interrogated while IDENT is active (for a set duration of time after you press the button in the cockpit), its reply data block will have the IDENT bit set. ATC computer systems notice this and cause the IDENT to be shown on the controller's radar display.

What does the controller see on their screen?

It depends on the type of radar system in use. Generally the radar target will either blink, its symbol will change (or "blossom"), or an indication such as "ID" will blink in the aircraft's data block (where the callsign is shown).
The AIM has some schematic pictures that show what an IDENT looks like on an ARTS-III scope (an older system still in use at some TRACONs). See Figures 4-5-2 and 4-5-3.
I will try to find some pictures of other radar systems such as those used at ARTCCs and newer TRACONs.

Does it depend at all on either the code or the mode the transponder is currently set for?

The code doesn't matter. The IDENT flag is an extra bit in the transponder's reply data that's independent of the 12 bits used for the code.
Obviously the IDENT will only be observed if the transponder is operating (not OFF or in a STANDBY mode). But the IDENT works in any mode used by normal civilian transponders (Modes A, C or S).

Wait, so does this mean simply pressing the button will not ident... the operator must HOLD the button until the transponder is next interrogated. That may be several seconds! That would explain why I always had so much trouble with this.

@DouglasHeld No, you don't need to hold the button. Pushing IDENT puts the transponder into a special mode for a specific amount of time. Any interrogation replies it sends during that time will have the ident flag set.

@Zaz I replaced all of the dead FAA.gov links. Unfortunately, I only found a PDF version of 7110.65 on the FAA's website now. If there's an HTML version somewhere, that would probably be a better link. For the AIM, I linked to faraim.org, which has an HTML version of the AIM, since I again only found a PDF version on the actual FAA website.

Where is the 18 second IDENT time duration specified? I see the specifications in most transponders use this value however I have been unable to find an FAA or other governing body which specifies either a range or the 18 second value.

@TypeIA figure links are failing...

In what situations does ATC ask for ident, and is it ever required by procedure?

ATC asks a pilot to IDENT for identification purposes. If used, it usually on first (secondary) radar contact. Identification can be done by other means as well, for example by identifying turns, correlation to reported position etc.
See this FAA document for how radar identification can be done by means of IDENT or otherwise.

What does the transponder actually do in this mode?

The transponder will, for a duration of 18 seconds after IDENT is pressed, change its reply to radar interrogations.
If the interrogation is a Mode A or Mode C interrogation, an additional pulse is appended 4.35 microseconds after the last framing pulse of the standard Mode A/C reply. This is called the SPI (special purpose identification) pulse.
If the interrogation is a Mode S interrogation (Uplink Format 4,5, 20 or 21) and the transponder is a Mode S transponder, the Flight Status field in the reply will be set to indicate that Ident is pressed.
If the Mode S transponder is transmitting ADS-B as well, the Surveillance Status field in the position messages will be set to indicate that Ident is pressed. This is also done for 18 seconds after the IDENT button is pressed.

What does the controller see on their screen?

The controller will see an indication that the aircraft is IDENTifiying. In what form the indication is presented depends on the ATC system. For example, the data block may be flashing, boxed, or (partly) highlighted.

Does it depend at all on either the code or the mode the transponder is currently set for?

No, IDENT is independent of the Mode A code (squawk code) set in the transponder.

From wikipedia (emphasis mine):

All mode A, C, and S transponders include an "IDENT" button, which activates a special thirteenth bit on the mode A reply known as IDENT, short for "identify"". When ground-based radar equipment receives the IDENT bit, it results in the aircraft's blip "blossoming" on the radar scope. This is often used by the controller to locate the aircraft amongst others by requesting the ident function from the pilot, e.g., "Cessna 123AB, squawk 0363 and ident".

In my limited experience, the time I had to IDENT was in crowded traffic so the controller could identify me. How it was explained to me (and I can't find an exact description for blossoming) was that the blip on ATC's screen that is you will "pulse", sending out concentric circles for a few seconds. It makes it very obvious which craft is sending out the IDENT. This article gives a brief, slightly different description:

When you push the IDENT button, it adds an extra pulse to your replies that causes your target on the controller's radar scope to change appearance. (It either "blooms" on an approach control radar screen, or has a flashing "ID" on a center radar screen.) Controllers sometimes use this to help find your target, or to make sure the target they think is you really is.


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Why does the US use two types of aerial refueling?
There are two main types of aerial refueling, the boom and the drogue. The US uses both, while most other countries use just the drogue.
How are they different and why does the US use both?

Fun fact, the first air to air refueling was done when a wing walker with a 5gallon jerry can strapped to his back stepped from one biplane to another.

related https://aviation.stackexchange.com/questions/51692/why-are-flying-boom-tankers-restricted-to-a-single-boom

The flying boom method uses a rigid boom which is "flown" by an operator in coordination with the pilot in order to make a connection with a receptacle on the second aircraft. The boom is retracted when not in use. Though it can do damage if it comes in contact with an aircraft, the operator can quickly move the boom if needed.

The probe and drogue method uses a trailing hose with a basket on the end. Pilots will guide a probe on their aircraft into the basket to connect with the hose. The hoses are retracted when not in use. Though the crew has less control of the drogue, which can still cause damage, there is less danger to the aircraft than from a boom, and it could allow the aircraft to stay further away from the tanker.

The boom method was developed early on in the Cold War, when Strategic Air Command had many large bombers that needed to be refueled. They needed a method that would transfer fuel as fast as possible in order to reduce the time it took to transfer the large amounts of fuel needed. Because the boom is rigid, it is capable of a higher transfer rate. The importance of bombers meant that SAC got their way, and the KC-135 fleet was equipped with booms. Today, almost all US Air Force planes capable of aerial refueling use the boom system.
The US Navy on the other hand, along with most aircraft in other countries, uses the drogue system. Though this method is not capable of the high transfer rates of a boom, it is much more flexible. A refueling aircraft may only be fitted with one boom, and this requires extra design work and integration with the fuselage. The drogue system, on the other hand, is more simple, and does not need a dedicated operator. This means that it can be more easily fitted to the refueling aircraft, even other fighter planes, and larger aircraft can have multiple drogues.
Another point is that smaller aircraft such as fighters are not capable of receiving fuel at the full rate the boom provides, so they have less benefit from the system. Although a boom can have a drogue adapter attached, this still commits the tanker to one aircraft at a time and one type of system per flight. Also, aircraft such as helicopters cannot use a boom system.
When the Air Force was looking at a replacement for its tanker fleet in the early 2000s, a study was done to determine what standard the new tanker should support.
The report suggests some options for the best combination of methods. In the end, the Air Force selected the KC-46 for the contract, which will include both the boom and the drogue options for refueling. This will provide better support to aircraft from other branches or countries, while maintaining support of the existing aircraft that use a boom.

The basket can actually do tons of damage. Its very heavy and made of steel. Even a glancing blow has been known to do 10's of thousands of dollars worth of damage. Basket strikes are almost always a class charlie mishap. Oh yes, and you should add a blurb in there about the Navy having much more competent pilots. Other than that, spot on.

@RhinoDriver thanks for the info, always good to have a first hand account. I will incorporate that.

Another reason the Navy kept the hose and drogue system is interoperability with foreign forces, most of which still use it. And of course KC-10s (and I think the -135 as well) can be fitted with underwing pods to refuel using probe and drogue.

@jwenting Some -135s can be fitted with pods; it also supports a metallic drogue adapter on the end of the boom (which according to Wiki is even more of a pain to deal with than a regular drogue). The -10 doesn't even need underwing pods (although some can take them); it has a roughly-centerline hose-and-drogue built in. As for foreign forces, while foreign-built aircraft all use probe-and-drogue, foreign forces with USAF-targeted planes (e.g. F-15, F-16) need to use booms to refuel them.

@RhinoDriver I have a sneaking suspicion that last part of your comment may be a tad biased. ;)

@RhinoDriver: According to Wikipedia, only the baskets used on boom-drogue adapter units are made of steel; the ones for ordinary probe\drogue assemblies are canvas.

Seem to have left out the need for a probe with the drogue method. That probe comes with penalties for weight, aerodynamics, vulnerable to damage(on ground and combat), and is limited to mount points located in the pilots visual field.

It may be easier for a pilot to move the basket rather than the plane.  With today's technology, a simple joystick to "fly" the basket might be better than trying to chase it with your aircraft.  Even simply stabilizing the basket might be extremely helpful.  The drouge could also be something similar to auto stabilizer "safe" function chip now found in R/C aircraft.  Easily small enough for any refueling system.

Another reason the Navy went with probe and drogue is that the drogue supply connector can be mounted on aircraft that can launch and recover to a carrier, allowing refueling capability anywhere a carrier can go, rather than within a few hours flight time of a land base with military jet fuel.  Less lead time needed for a refuel, fewer crew tied up and much lower cost for the tankers, too.

@RobertDiGiovanni, per my other comment, I don't see how that would work.  The pilot of the receiving aircraft is supposed to use his right hand to fly his aircraft, and the left had to operate a joystick that controls the drogue?  What about the throttle?  The current system has worked for decades now, there's nothing that needs fixing with "today's technology"...

Further examination might show the AF refueling needs to be more logistical involving very large aircraft,, while the Navy refueling is more like trying to refueling many smaller fighters protecting a carrier at once ASAP. And, who knows, refueling drones with perhaps 5 baskets or more may be in the cards.  May not need a boom operator for each drone.

The US Air Force and the US Navy have a very different history, so they approach things quite differently and feel their respective way of doing things is superior.
The US Air Force started as the Aeronautical Divison of the US Signal Corps and later became the US Army Air Forces (USAAF), and it inherited the traditions from the Army: Discipline and strict adherence to procedures, basically what was started when Baron von Steuben trained the first American soldiers.
The Navy used to give their commanders much more freedom: When away on a ship, the captain had to decide for himself what the best tactics would be. This can still be found in the Navy's flying forces today: Pilots are much more encouraged to think for themselves instead of following detailed procedures.
Therefore, both will instinctively reject what the other prefers: When the Air Force selected the F-16, the Navy had to prefer the F-18. And so on.
For aerial refuelling, it makes indeed more sense to use the boom technique when large amounts of fuel need to be transferred to big bombers. If you need to top up fuel for a number of smaller aircraft, however, the drogue-and-chute system is the better choice. The mutual competition between the Forces has only helped to cement this diversity.
The advantages of the boom over the drogue technique are:

much higher transfer rate. This is the reason the boom method was developed to transfer lots of fuel to SAC (Strategic Air Command) bombers.  Other nations' bombers and large aircraft prefer the probe and drogue method.
Maneuvering is done by the boom operator. While the drogue method requires the recipient to manoeuver his aircraft's probe into the center of the drogue, the boom method requires the recipient only to fly in formation with the tanker while the boom operator does the rest.
There are fewer boom operators than pilots, so the boom operator is likely to be more experienced in the refuelling process.

The disadvantages are:

The tanker needs to be a big aircraft which can afford to include a boom operator compartment. There is no possibility for buddy-buddy refuelling (except between two tankers).

Also most aircraft, including many of the USAF, can not accept fuel at the rate that a boom can provide, so much of the extra capability goes unused.

Retrofitting existing aircraft for boom refueling is much more complex than mounting a fuel tank with a hose and drogue at its end to an aircraft.  Unless the aircraft in question is a large aircraft, the retrofit to boom is virtually impossible.

the navy investigated the F-16 seriously, came (IMO correctly) to the conclusion that the required modifications would leave it too heavy, too slow, incapable of performing its mission. So they went with the F/A-18 instead, which was based on the YF-17 that lost the USAF competition and proved to be better suited for turning into a carrier aircraft.

@jwenting: I remember the discussion well: The Navy reasoned that the F-18 must be more capable, because hanging it full of ordnance would affect performance less that doing the same with the F-16. Yeah, right. (This only shows that the F-16 has cleaner aerodynamics). The same cherry-picking of arguments went on with the number of engines.

2 engines for an aircraft spending a lot of time over the sea is a good idea. As was shown even during flight testing of the YF-18A when a fan blade cracked, causing one engine to have to be shut down. The aircraft returned to base safely, a YF-16 would have crashed. AFAIK main concern with the F-16 for the navy apart from the engines was the amount of work needed to make it carrier capable, and the penalty to the performance that would cause.

Indeed, the F-18 E/F/G are the best carrier aircraft the Navy has ever flown.  Several design features make it incredibly reliable in the carrier environment, and I would definitely place it well above a modified viper for this flight environment.  The engines are incredible, throttle response at the boat is the best its ever been in a carrier aircraft, the flight controls are phenomenal (especially low speed control) and it can carry a larger payload, has better radar, is more upgradable--the list goes on.  The viper is great at what it does, but not for carrier ops.

@RobertDiGiovanni, I'm not sure what kind of basket stabilizing features you envision, but short of adding a gyro and computerized flight controls I don't see how it could be improved.  Even then, turbulence will make it bounce around.  It's at the end of a long flexible hose after all...

@MichaelHall cameras these days have remarkable stabilizing features.  Along those lines.  (and evaluating my perhaps overly cavalier holiday comment).  It might be that AF planes (transports and bombers) need such gargantuan amounts of fuel, the single boom is more favorable. Happy Near Year anyways.

@RobertDiGiovanni, I understand, but you'd have to have something optically lock onto the receiver aircraft and be capable of extremely rapid adjustments to maintain relative stability.  I think such a system would be extremely complex, expensive, and unnecessary in all but the most extreme cases.  I've fought the basket many times, but was never unable to get gas when I really needed it.  Simple works.  And Happy New Year back at you!

The Navy carriers also need to be self sufficient and so need to be able to launch their own tankers from their deck. Boom tankers are too large for this so the Navy relies on the buddy buddy system where one F-18 is the tanker and trails a hose for his buddy to refuel from.

The chief reason for this was fuel transfer rate vs flexibility for tactical aircraft.  The flying boom, as pointed out by others here offers a faster fuel transfer rate and makes it more suitable for strategic operations ie refueling large bombers en route to their targets, as the USAF had largely utilized their tankers for.  That being said flying booms require a specialized aircraft for that role whereas probe and drogue system can be employed much more flexibly on a wide range of airframes.  The Navy and other NATO nations found this more convenient as a means to provide gas for tactical aircraft during operations as fighters are an almost universal Air Force commonidy and many nations were not equipped with dedicated tankers and strategic nuclear bombers.  The Navy liked the probe and drogue system as well due to the limited space available on carriers for a fleet of tanker aircraft and it is more convenient for an exiting fighter or attack aircraft to use “buddy stores” - external tanks and a hose and reel pod to provide extra gas for the air wing.  Cold War tactical operations, Operation Desert Storm and joint military operations in the years since then have made the USAF adapt its tanker fleet to accommodate both types of refueling systems.  KC-135, KC-10 and the new KC-46 aircraft are equipped with both the flying boom as well and the probe and drogue refueling systems now.

Also, Navy aircraft can not trap aboard the carrier if there is a boom underneath the back of the aircraft (even if retracted).  With the compression of the landing gear from the force of the landing, that boom would get damaged or ripped off or caught in the wires which can also damage the aircraft, aircrew, deck crew, deck equipment, other aircraft on the deck, etc etc.

this is a nice and interesting comment but it does not answer the OP question.

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review


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What is the "area rule"?
The "area rule" states that the supersonic drag on an aircraft is related to the total area on part of the aircraft. What is the exact definition of the rule? Why does this happen?

It's related to the shape of the curve of cross sectional area as plotted along the longitudinal axis of the aircraft. In other words, let the x-axis be the fuselage station from the nose of the aircraft to the tail of the aircraft and let the y-axis be the cross sectional area of successive planes along the longitudinal axis of the aircraft and at 90 degrees to that axis. If a part of that curve has a sudden change of slope relative to the curve before and after that part, the area rule has been violated, and you will have greater transonic drag rise than if the curve was smoother.

I'm not an aerodynamicist so I'm not competent to write up a full explanatory answer, but I can point you to the Wikipedia article which has a lot of good information.

Also keep in mind that there are at least two different area rules: one for transonic flight and one for supersonic flight. For supersonic flight the algorithm is far more complicated—as instead of normal sections you must track the cross-sectional area of the aircraft across planes at the Mach angle $\mu=sin^{-1}(1/M)$.

You might have heard of the elliptical lift distribution which minimizes induced drag for a given span and a given lift. Area ruling is similar, but here it is about the lengthwise distribution of cross-sectional areas of a plane, and it minimizes wave drag at Mach 1.
The first picture is from a Junkers patent of 1944: Junkers found out in 1943 that transsonic drag can be minimized by shifting the parts of an airplane around. In this case it was the EF-122, a research project which became the Ju-287. The easiest bodies to shift were the engine nacelles, that is why the end product has such odd engine locations. The drawing shows several shaded cross sections along lines which carry Roman numerals: Plot the sum of all areas along each line over the aircraft's length coordinate, and you get a distribution which must be optimized such that it has the maximum area for the smallest gradients.

The next picture shows how those plots look like. This is from a different source, so it shows generic wing-body combinations. On the left is an airplane with a fuselage shaped like a Sears-Haak body. A Sears-Haak body is an optimum shape for the biggest volume with a given wave drag and was discovered by Wolfgang Haak in 1941 and again by William Sears in 1947. The wings come on top and mess up the area distribution by creating a bump in the middle.
The right plot shows what can be achieved by taking all parts of the airplane into account. The difference is shown in the small center plot.

How this is done: Take away from the fuselage what is added by the wings. The result is a fuselage wich looks like a coke bottle. You can also add Küchenmann bodies; older airliners have plenty of them. Look especially at fuselages like those of the Blackburn Buccaneer, or  T-tails like those of the Illjushin Il-62 or the Tupolev Tu-154. Today, things like engine pylons or flap track fairings are used as Küchemann bodies to smoothen out the lengthwise area distribution, and if you look carefully you will see the coke bottle technique on the fuselage around the horizontal tail of many airliners. They don't fly at Mach 1, but close enough that area ruling makes a difference already.
The dawing below shows the general concept and the magnitude of drag reduction achievable:

may I ask if you could add a more explicit picture of what a Küchenmann body is?

Wikimedia has a collection of pictures: http://commons.wikimedia.org/wiki/Category:Anti-shock_bodies

ah, those! had no idea they were called that way. Thanks!

In essence, the area rule is a general rule of aircraft design which prohibits abrupt changes in a aircraft's cross-sectional area from one point to another on it's longitudinal axis, since these would create areas of low pressure in the surrounding airflow that would literally hold the aircraft back, reducing it's speed through form drag. The area rule is sometimes referred to as the "supersonic area rule" since it has more significance at transonic and supersonic speeds.


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Do pilots keep light in the cockpit during long night flights?
During long times of the cruise flight, is the internal light normally switched on or off inside the cockpit?
Knowing it is generally dark in the night would make a smartphone-based night time logger easy to write.

Sounds like an XY problem to me.   ;)

Why would a pilot need a smartphone to tell them when it is nighttime, or dark?  It sounds like you're trying to solve a problem that doesn't exist.  Aircraft already have extensive data logs.

Related (maybe the motivation for this question?): https://aviation.stackexchange.com/questions/79191/how-should-night-time-be-calculated-when-it-is-not-night-during-the-entire-fligh

What information is to be logged, exactly?

If sunset happens in the middle of the flight, I assume, only time after sunset should be logged as a night time. By sensing where it happens, a phone would be able to produce the two log entries, day flight duration and night flight duration. Otherwise this cannot be done automatically without knowing the flight path, but GPS sensor of the typical consumer mobile device time to time just does not work in flight.

@h22 I'm skeptical that a photosensor on a pilot's phone is the best way to determine sunset for that purpose. Twilight can extend for quite a long time after sunset, depending on latitude and time of year (and, for that matter, weather.) Also, things that are not actual sunset could probably trigger this (e.g. flying under or through a cloud or in the shadow of a mountain. Or your phone being placed with the photosensor covered.)

@reirab a secure mount will keep the phone available for the pilot's use at all times and prevent accidental detection of night when the phone falls under the seat.

@J...: Some aircraft have extensive data logs, if for instance they're commercial airliners.  Others, like my Cherokee, have no logging at all.  (Other than the Hobbs.)

White lights are usually off in the cockpit in night flights to protect pilots' night vision. Red lights are sometimes used as these do not impact night vision.
One of the challenges you'll run into is knowing when it's night, as the start of night varies by latitude and the exact time. Phones generally set their time from the mobile network, which may be absent on long flights, especially those over water. A pilot flying a jet over the pacific may may have a phone that is hours off the local time.
Putting my (albeit old and tattered) mobile developer hat on I would suggest you look for ways the phone can tell you whether it thinks it is night or day and use that, plus a manual selection for day/night mode. Look at the Android UIModeManager as a way this could be done native.

I guess you can rely on the phone's integrated GPS to know exactly where and when you are, then deduce from there if it's time for red or white lights.

Sure @DiegoSánchez, with some completely doable calculations. There's probably code out there in the public domain to do it. However, you might be able to make it much easier: Q: 'Phone, is it nighttime?', A: 'Yes'

I agree with the first paragraph for VFR flying, but I know a few long-haul airline pilots who say they do tend to keep the cockpit lights on in cruise to help stay awake, unless there is something interesting to look at. Of course they need to be off for takeoff and approach. So I think the answer really depends on what type of flying it is and which segment they are in.

If there is no enforcement, I would rather log all my times in UTC. Looks like some pilots are indeed doing this: https://forums.qrz.com/index.php?threads/logbook-time-enter-utc-or-local-does-it-match-the-other-contacts-entry-time.380167/.

What problem are we trying to solve that a phone app will remedy?  Are modern humans incapable of differentiating between day or night to a degree that is significant enough to decide whether to log one over the other?  If you are on the razor's edge of having what you need that is one thing, but plus or minus a couple tenths of an hour would probably average out over time.

@MichaelHall My understanding is that the OP wants to prevent flashing a bright, white screen to the pilots if they fire up the application during the night. I would expect a trained pilot to be able to tell day from night, yes, but this little help would indeed be a nice touch as it would avoid killing their night vision. I wish mi car was half as smart when driving in the night.

@Diego Sanchez, Interesting, I didn't interpret the question that way. Read the second sentence again, I think he is asking to know if measuring the ambient light in the cockpit would be a reliable way for the app to recognize it was night. Leaving a white light on would mess a simple algorithm up.

@MichaelHall I guess you might be right.

@GdD Most smartphones require internet to answer questions like that. Also, you may end up with a surprising answer. I just asked Siri whether it is night time (it's currently the middle of the day for me) and she answered with a weather report...

That's not what I mean @Nzall, OS based mechanisms won't require internet access.

If you are asking about logging night VFR -time, you are not concerned with lighting conditions. Night time is logged according to regulation based definition of night (at least EASA), tables for night time are usually available from local aviation administration websites.
Your logger simply needs to know your position, date and time, and refer to the official table or definition of night if table is not available. EASA defines night as
" ‘Night’ means the period between the end of evening civil twilight and the beginning of morning civil twilight or such other period between sunset and sunrise as may be prescribed by the appropriate authority, as defined by the Member State."
Not quite as informative as one might like, but this is how thing work in avaition...

That's because EASA is not actually a civil aviation authority and the member states may be using different definitions. But civil twilight is precisely defined, and alternate is certain time after sunset/before sunrise (1 hour in the USA), which is also precisely defined. You just need to keep track of the definition by country.

Good clarification Jan

… now thinking of it again, there is one point I don't know though – does the definition of the country that issued your license, or the country where you are flying count (I would expect the former, actually).

Oh, that's a tough one. You obviously operate under the rules of the country you fly in, but logging time... hmmm... if the country you fly in specifies it's night VFR, I would log it as such, even if my "home rules" did not consider it a night. You excercises the qualifications of your licence in accordance with the local regulations.

To strictly answer the title question, at least in GA operations, we typically fly with the cockpit lights off to avoid ruining our night vision. We do sometimes use red lights to see stuff in the cockpit, though.
However...
It's unlikely that a photosensor will be very useful for determining which hours count as day vs. night. Lots of factors will affect the relationship between time of day and received light on a photosensor in the cabin. These include, but aren't limited to: position, altitude, heading, orientation of the phone, latitude, time of year, weather, etc. This is compounded even more by the fact that what counts as "night" varies, not only by country, but even within the same country depending on the purpose in question!
For example, according to this AOPA (Aircraft Owners and Pilots Association) article, the US FAA definition of 'night' for purposes of logged flight time for private or commercial certification is the one in 14 CFR 1.1:

Night means the time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the Air Almanac, converted to local time.

It's highly unlikely that you'll be able to figure that out with a photosensor on a phone in the cabin. You'll instead need the aircraft's location and time (and the formulas for figuring out when civil twilight starts and ends.)
However, for purposes of being 'night current' in order to be allowed to carry passengers at night, the definition of night is defined differently in 14 CFR 61.57(b):

the period beginning 1 hour after sunset and ending 1 hour before sunrise

Indeed, it turns out that the FAA uses several different definitions for different purposes, as described in detail in answers to the question How does the FAA define day and night? And that's just the U.S.
So, what you'll really need to solve your problem is:

Aircraft position
Time
The relevant rule for whatever purpose you're wanting to know about day vs. night.

Thankfully, the first two should be easy with a modern smartphone. At least for the U.S. definitions, local time should be irrelevant. Your phone's system time (in UTC) and position should suffice, so crossing time zone boundaries shouldn't matter.

Ironic that the legalistic definitions fail to capture the one parameter that differentiates the two in any meaningful way...

@MichaelHall Are you referring to sun disc angle relative to the horizon? If so, the legal definition of "civil twilight" does encompass that, as do the definitions of "sunset" and "sunrise." Granted, I would agree that "1 hour after sunset/before sunrise" is a silly legal definition, since that can result in a dramatically different amount of lighting depending on latitude and time of year.

No, I am referring to the "dramatically different amount of lighting" between day and night.  I understand that there is some subjectivity during the transition period, therefore it needs to be defined somehow.

@MichaelHall Defining it in terms of actual lighting would be really hard to do and almost impossible to enforce. It would vary from day to day based on weather and also from one nearby location to another based on terrain (e.g. it gets dark significantly earlier at the base of the Eastern slope of a mountain than at its summit or on its Western slope.) Astronomical definitions, on the other hand, result in times that are consistent within an area and don't depend on weather (and, thus, can be computed as far in advance or retrospect as desired.)

I agree with you to up to a point, but if I fly 2.0 and it gets dark roughly halfway through the flight I log 1.0 day, and 1.0 night.  That’s it, that’s what I do.  And I don’t lose a minute of sleep worrying about accuracy of astronomical calculations or wondering if I might get audited.  Maybe I’m just an old fashioned rogue that way...

@MichaelHall Agreed and I do pretty much the same thing. I'm just saying that using amount of light to set the legal definition would be tough because, unless you have a photosensor on a gimble that remains aimed at the sun, it's subjective to measure and, even if you do have that, clouds, haze, mountains, etc. will mess up accuracy. "Night starts at a time when the solar disc is x degrees below the horizon and is published in the pilot's almanac" is much less subjective than "night starts when it's dark," even if pilots really use something more resembling the latter in normal practice.


text:
Who was the oldest pilot ever so far?
Based on any historical records, who has been the oldest pilot ever? "Pilot", I mean, somebody who managed to take off, return to the airport and land without  help. That much is probably not possible without training; would still be interesting to know if this pilot had a valid or maybe expired license.

Cole Kugel, 105

Born March 14, 1902, Cole Kugel, a resident of Longmont, Colorado, USA is the oldest qualified pilot. He was born one year before the legendary Wright Brothers took to the skies, and has been flying ever since. Kugel earned his pilot's license in 1945 and flew in 2007 for the last time, aged 105, piloting in the company of local veterinarian Lynn Ferguson in his beloved 1976 Cessna 182.

http://www.guinnessworldrecords.com/records-1000/oldest-qualified-pilot/

2005 Photo: Daily Times-Call | Hunter McRae

He's been flying ever since he was born? Impressive.

I showed this to my father and he said, hmmm, only 23 more years to pass him, the old coot.

@fooot Not only he was flying since he was born, but even before Wright Brothers invented an airplane.

Apparently the oldest person to earn a pilot's license was 87 at the time - some interesting "oldest pilot" records over at Guinness!

@voretaq7 link is N/G